Caustic leaks and the solution freezing every winter worried operators at the direct filtration Columbia River Water Treatment Plant in Richland, Wash.

A backup system of automatic valves on the chlorine cylinders met regulatory requirements, but the facility was storing 22 tons of chlorine gas in the middle of a residential neighborhood. In case of a ruptured cylinder, the staff wanted the emergency scrubber operational.

“The unit was the best technology available when we installed it in 1996,” says John Finch, water manager. “But twice in four years, cracks in the fiberglass vessel caused leaks, and we sent it out for repairs.” Besides the safety issue, the 20 percent sodium hydroxide solution froze at 13 degrees F, making the scrubber inoperable for weeks in winter.

In 2006, the city built its third water treatment facility, and engineers specified an emergency dry scrubber from Purafil. Finch saw it as the solution and solicited a quote for a 1-ton Purafil dry scrubber from Treatment Equipment Co.

“Somewhere in the process, Purafil engineers determined they could retrofit our scrubber using their Chlorosorb ultra-dry chemical media,” says Finch. “That decision saved the city $70,000 based on the budgeted amount for a replacement unit.”

Stress fractures

The 30 mgd (design) plant, serving 50,000 people, treats water from the Columbia River with coagulation, chlorination, and filtration. The chlorine facility can disinfect before and after filtration depending on surface water conditions. It uses 35 tons of chlorine annually. If a cylinder were to rupture, a caustic pump would send solution through spray bars and onto plastic pucks, while a 7.5 hp, 3,000 cfm blower pulled the gas from the building and through the media.

Operators checked the concentration of the caustic in the tower annually and monitored the area regularly for signs of white powder on the grating above the secondary containment tanks. When the sodium hydroxide residue appeared, it was time to repair the vessel. “We weren’t dealing with catastrophic breaks, but caustic should not be leaving the vessel,” says Finch.

To prepare the tower for shipment, operators drained the solution in the reservoir and reaction chamber into two portable 1,200-gallon storage tanks. The city sent its sewer cleaning crew to vacuum out the pucks and a boom truck to load the vessel onto a trailer for transport to a local vendor. Downtime was almost 30 days. A few months after the tower returned, the hairline cracks reappeared, and they kept enlarging until it was necessary to repeat the repairs.

Cooperative effort

“The retrofit was a big job,” says Finch. “It wasn’t like changing a light bulb. If municipalities don’t have really qualified staff and expertise, they shouldn’t tackle it. But if they do, they can save a lot of money.” As a full-service city, Richland owned everything needed for the project. 

Finch appointed Brent Andrews, a lead plant operator, as the liaison between the city and Purafil engineers. He selected Dennis Crater, a maintenance person, as project supervisor in charge of stripping out the vessel and installing the new components. Whenever questions arose, Andrews emailed Purafil and scheduled a conference call. “The engineers were really helpful and easy to work with,” says Finch. “But having one person in charge of communications on our end made everything flow smoothly.”

Workers cleaning out the tower followed confined-space entry procedures, ventilated the enclosure, and wore gas meters. After draining the liquid caustic, they entered the vessel to remove the spray bars and pipe before sucking out the pucks. They then cut out the internal fiberglass walls and the barrier between the reaction chamber and reservoir. A second crew removed the blower and recirculation pump. The work took a week.

Conversion package

The retrofit included an aluminum framework, a prefabricated wall, a 30 hp, 5,000 cfm blower, a control panel, media screens, and 13 cubic yards of nontoxic dry-scrubbing pellets shipped in Super Sack containers.

Workers installed 12 frame dividers in three rows of four, then secured the screens to them. Finally, they dumped the media between the screens. The pellets, which permanently transform gases into harmless solids, are immediately available for instantaneous reaction regardless of the gas load rate.

The chlorine leak detector in the chlorine building connects to a programmable logic controller (PLC). If a leak is sensed, the scrubber comes on automatically, and the control panel turns on the blower. The scrubber discharges less than 25 ppb chlorine gas.

Except for the need to send an annual media sample to Purafil to test its efficacy, the system is maintenance free. “We don’t worry anymore,” says Finch. “Even when it’s 10 below zero, we know the scrubber will work if needed.”